This application describes and claims certain improvements in the basic electrochemical cell disclosed in U.S. Pat. No. 3,791,871 issued February 12, 1974.
The basic mechanism of operation of the cell described in the aforementioned patent is incorporated by reference in this application. Briefly, the cell utilizes a reactive metal anode highly reactive with water and spaced from a cathode by an electrically insulating film formed on the anode in the presence of water. The anode and cathode are immersed in aqueous electrolyte. In the embodiment shown in the aforementioned patent, the anode is formed of an alkali metal such as sodium or lithium and, during oeration of the cell, the electrolyte is a liquid solution in water of an alkali metal hydroxide. Alloys and compounds of the alkali metals and other reactive metals should be equally feasible for use as the anode, however, provided they are substantially as reactive with water as are sodium and lithium and further provided, in common with sodium and lithium, they form an insulatingfilm in the presence of water. The electrolyte is preferably an alkali metal hydroxide of the alkali metal utilized as the anode since such hydroxide is naturally formed during operation of the cell and hence automatically regenerates the electrolyte during operation. However, other alkaline electrolytes can be used to initially start up the cell or even during operation of the cell provided they permit the required anode-cathode reactions. Illustratively, potassium and ammonia hydroxide and alkali metal halides are feasible. After start up, these electrolytes will become replaced by the hydroxide of the anode metal unless subsequent additions of these electrolytes are made during operation of the cell.
Operation of the cell described in the aforementioned patent involves the following reactions, which, for illustrative purposes, utilize lithium as the ractive anode and lithium hydroxide as the electrolyte:
Anode Reaction
1. Li.fwdarw.Li.sup.+.sub.(aq) + e electrochemical dissolution
2. Li.sup.+.sub.(aq) + OH.sup.-.sub.(aq) .fwdarw. LiOH.sub.(aq)
3. LiOH.sub.(aq) .fwdarw.LiOH.sub.(s) formation of insulating film on anode
4. Li + H.sub.2 O.fwdarw.LiOH.sub.(aq) + 1/2H.sub.2 direct corrosion/parasitic reaction
Cathode Reaction
5. H.sub.2 O + e.fwdarw.OH.sup.- + 1/2H.sub.2 reduction of water
Where (aq) represents an ion dissolved in water and (s) represents a solid salt.
Reactions (1) and (5) are necessary for the generation of electricity. Reactions (2) and (3) serve to produce the porous insulating film which forms on the anode and protects it. Electrochemical reaction (1) occurs at the base of the flooded pores, the metal-solution interface. Simultaneous with the formation of the film, lithium hydroxide crystal sites at the film solution interface dissolve into the bulk electrolyte. In order for the electrochemical reaction to proceed at a given constant rate, a steady state situation must exist whereby the electrochemically produced film dissolves into the electrolyte at the same rate as it is formed. Therefore, the electrolyte must have the capacity to dissolve solid salts from the anodic film-electrolyte interface simultaneously with the formation of the salt at the lithium-film interface. If the film dissolves more slowly than it forms, it becomes increasingly thicker and less porous and the electrochemical reaction rate slows down and can approach zero. If the film dissolves more rapidly than it is formed, then a higher reaction rate will result due to the thinner, more porous film. Ultimately, the film could disappear and the lithium become unstable.
Reaction (3) requires a sufficiently high concentration of lithium hydroxide at or near the anode to cause precipitation of the film as solid lithium hydroxide on the lithium surface. If the electrolyte temperature is elevated or if the concentration of hydroxyl ions is low, then the liquid hydroxide of reaction (2) does not convert to the solid hydroxide of reaction (3). In the case where the cell is in an open circuit mode, the film is formed via the direct corrosion reaction (4). It is not known with certainty if the film which forms electrochemically is identical to the film which forms via reaction (4).
Reaction (4) generates no useful electrical current. Ideally, during discharge, there should be no reduction reaction at the anode surface so that the faradaic efficiency of the cell becomes 100 percent as all lithium is utilized in th generation of useful electrical current. Since water is required as a reactant only at the cathode, reaction (5), the problem becomes one of inhibiting the direct corrosion reaction (4) by reducing the chemical reactivity of water so as to inhibit the direct evolution of hydrogen at the anode without inhibiting hydrogen at the cathode.
As discussed in the aforementioned patent, it is necessary to maintain molarity of the electrolyte within certain limits for any specified operating condition. Since reaction (1) increases metal ion concentration of the electrolyte, molarity is adjusted by water additions to the cell. The excess electrolyte is either collected or disposed of into the environment. The cell, therefore, is particularly appropriate for a marine environment which can supply unlimited water and can absorb the reaction products. For nonmarine applications of the cell, it would be desirable to find an alternative technique for controlling molarity of the electrolyte.
A further disadvantage of the cell described in the aforementioned patent is its inefficiency at elevated temperatures. This is related to the higher solubility of the compound comprising the protective anodic film at high temperatures and also to the higher rate of the direct corrosion reaction (4) at high temperatures.